Somatic cell hybridization techniques have been used in order to locate the integration sites of DNA and RNA tumor virus genomes in virus transformed cells. Somatic cell hybrids between mouse 3T3 cells and peritoneal macrophages and human cells transformed by SV40, Adenovirus 5, Kirsten murine sarcoma virus, Moloney murine sarcoma virus, Rous sarcoma virus and Epstein-Barr virus have been produced and are being characterized. The hybrid clones are studied for the expression of viral structural proteins and tumor antigens, for the expression of viral genomes, and for their tumorigenicity in nude mice. Karyological and isozyme analysis of the hybrid clones is in progress to determine the chromosomal location of the viral genomes. BIBLIOGRAPHIC REFERENCES: Orkieszewski, K.G., T.A. Tedesco, W.J. Mellman, and C.M. Croce. Linkage relationship between the genes for thymidine kinase and galactokinase in different primates. Somatic Cell Genetics, 2: 57-62. 1976. Croce, C.M., D. Aden, and H. Koprowski. Somatic cell hybrids between mouse peritoneal macrophages and SV40 transformed human cells. III. Tumorigenicity of diploid hybrid cells. Science 190:1200-1202, 1975.